Translocations of the transcription factor AML1/Runxl account for more than 30 percent of acute myelogenous leukemia and 25-30 percent of acute lymphoblastic leukemia. Gene targeting studies have demonstrated that AML1/Runxl is essential for the establishment of the definitive, but not primitive hematopoietic system. More recent gene expression analyses suggest that Runxl may play a role at a developmental stage earlier than predicted from knock-out experiments. The goals of this proposal are to further define the earliest stages of hematopoietic development regulated by Runxl and subsequently to identify and characterize genes involved in these commitment steps. The experiments in the first aim are designed to define the precise stage of hematopoietic development affected by Runxl deficiency. The focus of the second aim will be to further investigate the consequences of forced expression of Runxl on the developmental potential of wild type ES cells. The third aim will identify genes that function at the developmental stage(s) regulated by Runxl. The results from these studies will provide new insights into the cellular and molecular events involved in the establishment of the definitive hematopoietic system and the role of Runxl in this process. These findings, in particular the third aim, will ultimately shed new light on the critical events involved in the onset of leukemia following translocations at the AML1/Runxl locus.